Transistor as an Amplifier Video Lecture - NEET

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please make sure that you have watched
all the videos to the part 26 before
going ahead with part 27 now let us talk
about transistor as an amplifier which
is a very important application of
transistor so when does and how does a
transistor act as and amplified when a
transistor acts as an amplifier why is
this plot again here that's because the
transistor acts as an amplifier only in
the active region right if you look at
this graph you can see that in the
active region V naught by V I is equal
to negative that means as V input
increases the output will decrease so
that means the output is out of phase
with respect to the input correct this
is what we can see from this graph now
we will discuss in detail and we will
see that how the voltage gets amplified
by an amplifier by the output voltage is
out of phase with respect to the input
so we will study all that now we also we
can also define the small voltage gain
of the amplifier from this plot what
would be the small voltage in that will
be nothing but a small change in output
voltage by a small change in input
voltage so basically when you give an
input signal let us suppose if I have an
input signal somewhat like this when
this input signal is applied to the
transistor what do we get as the output
let us suppose this is my transistor the
output is somewhat like this that means
the output is out of phase and also the
output is amplified so now our aim is to
see how does this transistor does this
amplification what does the strands
to do to give this type of an output
right so let us have a look at that so -
in order to see how does the transistor
act as an amplifier the circuit also
almost looks similar to that of the
transistor acting as a switch so for now
let us concentrate only on this much
part of the circuit we will talk about
the input signal and the output signal
later so considering this part of the
circuit again applying the hilltops rule
on both these circuits this circuit as
well as the circuit so applying the
applying kirchoff's rule what do we get
we get VB b is equal to ib RB plus vbe
this is what we get for the input
circuit and we get another equation VCE
is equal to VCC minus IC RC that is the
output for this for that is for the
output circuit right now we will see
what happens when I am AC signal input
is applied so for that we will quickly
remember the output characteristics of
the transistor we discussed sometime
back output characteristics of
transistor what do we see we saw that
the collector current increases with
increase in the collector emitter
voltage and then it attains a saturation
value and this collector current also
increases with increase in these days
parents like that that is what we saw in
the output characteristics of a
transistor right so what will happen so
what will happen in case of this circuit
so this is my circuit now let us suppose
now I apply an input signal say VI and
here I get
and output signal say V naught and this
is VCC and this is V V V right so what
will happen if I apply this input signal
now when the input signal is applied
when input signal is applied it will
change the value of this voltage so this
can either become v vb plus VI or it can
become v vb minus VI depending upon the
cycle of the input signal because i am
assuming that an input signal of this
kind is applied here so when it is on
the positive half of the cycle so it
will become v vb plus VI when it is in
the negative half it will become V V V
minus V a so the this voltage will vary
between these two values
right now this voltage will change when
this V baby changes what what will this
v VD change if you look at our previous
slide you will see that when V DD
changes it will also change
ib right that is the base current will
change now so this will change the base
current now from the output
characteristics of the transistor we can
say that if the base current changes is
going to change the collector current
now if the collector current changes
then again from the second equation in
the previous slide we can see that we
see he will change now VCE is nothing
but our output right so we saw that how
an input signal will cause a variation
in the output signal and this is how the
transistor amplifier and input signal
because when an input signal is applied
this changes the input voltage it
changes the base current which in turn
changes the collector current which in
turn changes the output and this
variation is seen as an amplification
produced by the transistor right now
should also ensure that it not only
produces an amplified output but the
output should also be an undistorted
output all right
so that means the biasing of the
transistor that means the values which
we select for VBB VCE ID all these
values should be proper so that the
output which we get is an undistorted
output so what does the transistor do in
order to amplify the input signal as
well as give an undistorted output so
let us look at that so for an
undistorted output transistor should
work at its operating points now you
must be wondering what is an operating
point that is what we will study in this
point now and in order to determine the
operating point load line is needed so
again rather surprised for you that
because you don't know what a load line
is at all so now we have in this slide
we will try to understand how do we
determine the operating point of a
transistor because if I if a transistor
operates at its operating point in that
is the output would be and undistorted
output right so let us understand the
operating point for that we should
understand the load line and for
understanding load line we need the
output characteristics of the transistor
so this is the output characteristic of
the transistor mathematically we can
write VCE is equal to VCC minus IC RC
this is the expression for this graph
right this expression indicates the
output circuit this is what we wrote
using Kirchhoff's law right okay so what
is load line load line is a line which
intersects the x-axis and the y-axis so
we have to draw a load line which will
intersect the success that is VCE as
well as it will intersect I see so now
if we want an axis to touch this axis
that is the VCE axis what I have to do I
have to make IC 0 so put I
see equal to zero so what do we get we
get VC is equal to VC C that means if I
draw a line such that it should cut the
x-axis at this particular value right
now if I want to cut it on the y-axis
then I'll have to put VCE is equal to 0
so if VCE is equal to zero what would be
the value of IC it will be VCC divided
by RC right so and using that we will
draw a line say I draw a line like this
it is not really straight but I just
drew it so then this point where it cuts
this axis would be nothing but VCC as
per this again the point where it cuts
the y-axis that is this point would be
nothing but VCC divided by RC so these
two values can be experimentally
determined because we can plot this
graph experiment in this graph has been
plotted experimental by changing the
value of base current the variation of
collector current with the collector
emitter voltages observed and then that
is plotted in graph so then you have to
know you will be knowing the value of
VCC in your experimental apparatus you
will also know the value of the
resistance which is selected which is
connected to on the connector side right
so knowing these two values you will
note these two points when you know
these two points you join these two
points and this line which you get is
known as the load line why is it called
load line because it depends on the load
which is connected in the circuit which
is the load connected in the circuit it
is the load resistance that is RC which
is the resistance connected on the
collector side right so this is the load
line now any
point O now if you look at this loads
line but what will you see you will see
that when you observe this portion what
do you see the current is maximum at
this point like so at this point let us
call this point as a and let us call
this point as D so we can say that at
Point a current has maximum value so
this is the saturation mode of the
conductor right now if you look at the
other point B what do you see here
current is almost dropping back to a
very low value current is almost
dropping back to zero so this is the cut
off mode so on the output
characteristics graph of a transistor we
can say that this region is the cutoff
region of the transistor and this region
is the saturation region of the
transistor so that means the load line
defines which is the cutoff region which
is the saturation region and which is
the active region so which is my active
region so the remaining portion is the
active region that is that a region of
the transistor so any point on this
active region will act as a suitable
operating point that means any point on
this lap load line within the active
region will act as a suitable operating
point so most of the times it is taken
somewhere around the midpoint of this
line so if you choose a point somewhere
here you will get an operating point and
if the transistor operates at this point
because whichever point you choose let
us suppose you choose this point so
there will be a voltage there will be a
VCE corresponding to this point there
will be a collector current
corresponding to this point there will
be a base current corresponding to this
point so if that
transistor is made to operate with these
values of voltages and current the
output which will which we will get will
be an undistorted output so this is the
concept of the operating point of the
transistor now operating point of
transistor plays a very important role
in amplification because if you do not
know anything about the operating point
the output which you will get will be an
extremely distorted output so there is
no point getting an output of that kind
so when a transistor acts as an
amplifier firstly it causes variation in
the output voltage how does it cause
that we saw in the previous slide when
the input voltage is applied is it
causes a change in base then it which in
turn causes a change in collector
current which in turn changes the output
voltage right secondly it gives an
undistorted output how do we get an
undistorted output by selecting a
suitable operating point so for that
what do we do we construct a load line
using the output characteristic curve of
the transistor okay so that is also done
now the third thing which the transistor
does is the phase reversal you saw that
when we gave an input of this kind we
got an output of this kind so that means
the output is 180 degree out of phase
with respect to the input so how does
the transistor cause this phase reversal
so now we will consider the entire
circuit of them transistor till now we
were considering the circuit till this
part so now let us consider this part
two and let us also see that we have two
additional capacitors now so why do we
have these capacitors so that the
capacitors can block the DC component of
the input signal so what is the purpose
of this input capacitor if this input
capacitor if the input capacitor is not
there in that is the input DC will not
be blocked as a result the base current
or the DC base current will change and
therefore the operating point will keep
changing because in the previous slide
you saw that the
operating point is dependent on the
value of the base current so if they
want this transistor to operate at a
particular operating point the value of
the DC base current should remain the
same so that is why we place a capacitor
here so that it can block all the DC
component and therefore IB can remain
constant then the operating point will
also not change now why do we have this
output capacitor where C naught so that
it can block any DC on the output
because right now when we are talking
about phase reversal we are talking only
about the alternating signals we are
only talking about the AC components so
for that purpose we have blocked the DC
components so that let the operator let
the transistor operate in its operating
point so that we get error and
absolutely undistorted output at the
same time we just get the output AC
signal and so that we can compare it
very clearly with that input AC signal
so that is our purpose here so in this
case again let us look at the input
circuit which is determined by the
expression V be V is equal to IV RV plus
V ve and output which is given by VCE is
equal to VCC minus IC RC now what
happens in the positive half cycle now
during a during positive half cycle of
the input signal what will happen V
input will increase as I told you this V
input will become right now it is VB V
then it will become V BB plus VI because
it is the positive half cycle so your
internet input voltage is increasing as
a result if this is increasing IB will
also increase so IB increases as a
result I see also increases and if IC
increases your VCE decreases that is the
output voltage decreases that means
positive input voltage gives rise to or
creative output voltage right now you
might be wondering what is this IB IC
now right now we are talking about the
AC conference because the DC components
are blocked so using this expression we
are just trying to see how will it
affect the output current now let us
talk about the negative half cycle what
will happen in the negative half cycle
this VI will decrease as a result the
base current will decrease which in turn
will decrease the collector current and
therefore the output voltage will
increase so therefore it will increase
so that is how we get a phase reversal
of 180 degrees right so now let us talk
about some of the games which are
involved in a transistor so the first
one is the DC current gain in case of an
amplifier so the DC current gain is
denoted by beta DC which is nothing but
output current by input current that is
IC by IB when we talk about AC current
gain AC current gain is nothing but the
change in collector current by the
change in base current let me talk about
the AC voltage gain which is generally
which we generally denote by a V that is
again output voltage divided by the
input voltage now what is this output
voltage V is equal to IR we know that so
this can be written as Delta IC into RC
divided by Delta IB into RB so this can
be written as Delta IC by Delta IB what
is it it is nothing but AC current gain
so AC current gain into RC by RB so what
is RC by RB that is output resistance by
input resistance so we can say this is
nothing but resistance gain right
so this is what is AC voltage gain next
one is the power gain of the amplifier
so this power gain can be written as
power gain can be written as output
power by input power what will be the
hour now we know that power is equal to
I square R so the output power will be
nothing but IC square into RC / IB
square into RB so from this we can say
Delta IC by Delta IB whole square into
RC by IB what is Delta IC by Delta IB
that is AC current gain square what is
RC by RB that is resistance gain so this
can be written as beta AC multiplied by
beta NC into resistance gain now in the
previous slide we saw that beta AC into
resistance gain was nothing but the
voltage gain so this becomes infiltrated
this right so these are some of the
games which are associated with a track
which associated with an amplifier okay
so that s where we conclude the topic on
transistors so I hope I was able to
explain you the basics of a transistor
however we did not go into very much
deep about transistors because that is
not included in your curriculum as you
go for higher studies you will study
more about transistors you will study
more about transistor biasing and more
complicated circuits using transistors
so let us try to solve few problems on
transistor thank you please visit exam
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